1. Field of the Invention
The present invention pertains to a charging circuit that performs charging of a prescribed load and to equipment accompanying this charging circuit, said charging circuit being used, for example, in a flash light device, a camera with a built-in flash, a measuring device using flash light, or a battery charger.
2. Description of the Related Art
The circuit shown in FIG. 12 is one example of a conventional charging circuit, and is used as a charging circuit for a built-in flash in a camera. This circuit comprises step-up transformer 100, power supply battery 101, main condenser 102 for flash light emission, FET (field effect transistor) 103 and diode 104. FET 103 is placed in series with primary coil L1 of step-up transformer 100, and pulse signals having a certain frequency are input from drive control circuit 105 to gate G of said FET 103. The periodic turning ON and OFF of FET 103 periodically turns ON and OFF the application of primary voltage El to primary coil L1 of step-up transformer 100 such that AC (alternating current) secondary voltage Vf is applied to secondary coil L2.
AC secondary current I2 output by means of the secondary voltage Vf is rectified by diode 104 and supplied to main condenser 102 such that main condenser 102 is intermittently charged. When charging voltage Vh of main condenser 102 has reached a prescribed final level, the output of said pulse signals by drive control circuit 105 is stopped and the charging operation comes to an end.
In the charging circuit described above, where one battery is shared for the charging circuit and for the operation of the camera main unit, if a sudden step-up of voltage is made to take place in a condition where the battery has been consumed and its voltage is relatively low, the battery voltage will suddenly decrease, which may cause problems in the supply of power to other circuits of the camera. Even if the battery is not shared between the charging circuit and the camera main unit, if load is applied suddenly in a condition in which the battery has been consumed, the battery voltage will decrease suddenly, which may cause a problem with the charging circuit itself. Conversely, if charging is performed slowly so that the load is light, charging must be performed over a long period of time.
The object of the present invention is to provide a charging circuit in which the charging time may be reduced and in which a sudden decrease in battery voltage during charging may be prevented, such that problems in the operation of circuits may be prevented.
Another object of the present invention is to provide equipment accompanying the charging circuit in which no problem occurs in the power supply to circuits other than the charging circuit due to a sudden decrease in battery voltage during charging.
Yet another object of the present invention is to provide equipment accompanying the charging circuit in which the charging time may be reduced and problems in the operation of the equipment due to a sudden decrease in battery voltage during charging may be prevented.